1. Field of Invention
The present invention relates to a diamond substrate and a method for fabricating the same, and more particularly, relates to a diamond substrate having a composite layer structure formed by diamond and SiC and a method for fabricating the same.
2. Related Art
Diamond has excellent properties, such as high light transmittance for light from deep UV light to far infrared light, high surface acoustic wave velocity, high thermal conductivity, high hardness, high radiation-resistance, good chemical inertness, and good insulation property, so that diamond has been widely applied to conventional cutting tools and abrasive tools. Recently, along with the development of the chemical vapor deposition (CVD), the application of diamond has expanded to high-frequency communication devices, heat sink and optical devices of optoelectronics, and diamond semiconductors, etc.
As a photoelectric semiconductor, the light emitting diode (LED) has been developed for over 30 years, and the red, green, and yellow LEDs are well developed. However, the development of blue LEDs was quite slow. Under the development of material and epitaxy, the brightness of blue and white LEDs has been significantly enhanced. At present, a sapphire wafer is mostly used as an epitaxy substrate in the fabricating process. A polycrystalline AIN film and a single-crystalline AIN film are sequentially formed on the sapphire wafer substrate to serve as a buffer layer. And then GaN is formed on the buffer layer, thus obtaining a preferable quality of GaN crystal, and thereby enhancing the luminous efficiency and stability of the LEDs.
The abovementioned conventional process still has many aspects to be improved, and till now many researchers have been devoted to improving the fabricating process. Some of them propose to use the aforementioned diamond with excellent properties to replace the sapphire substrate. However, though the lattice constant between diamond and GaN is matched better than that between the sapphire substrate and GaN, it is still difficult to form a single-crystalline GaN on the surface of a single-crystalline diamond film layer. Besides, due to the thermal stress existing between the diamond layer and the base layer and the internal stress caused by defects in the diamond layer, the diamond layer is easily deformed or fractured. Those mentioned above are problems difficult to be solved.